1. Field of the Invention
The present invention relates to a battery pack, which is formed by connecting a plurality of secondary batteries, and an electric bicycle in which the battery pack is installed.
2. Description of Related Art
In electric bicycles, various kinds of secondary batteries are used as power sources for drive or auxiliary drive. Among the above secondary batteries, a lithium ion secondary battery, in which charge and discharge take place as lithium ions move between positive and negative electrodes, is preferably used as a battery for a drive power source because the lithium ion secondary battery has the following battery characteristics: high energy density and high output power.
The following lithium ion batteries are known: a lithium ion battery that is in the shape of a cylinder around which positive and negative electrodes are stacked and wound via separators; and a flat lithium ion battery in which positive and negative electrodes are stacked via separators.
Among the above lithium ion batteries, the flat lithium ion battery is preferably used as a battery for a power source of a device driving motor or the like because it is easy to increase capacity per unit battery by increasing the number of positive and negative electrodes stacked or by increasing the areas of positive and negative electrodes.
As for a unit battery of the flat lithium ion battery, it is possible to make effective use of the high energy density that the lithium ion battery has by covering battery elements with a film casing material.
The film-covered battery is used in various ways. For example, what is proposed in Patent Document 1 (JP-A-2007-257901) is a battery pack for an electric bicycle that uses a film-covered battery.